Paging method and apparatus for wireless communication system

ABSTRACT

A method of operating an infrastructure node in a wireless communications system is provided. The method comprises detecting that downlink messages for a communications device to decode should be transmitted by the node in one or more of a plurality or temporally spaced paging occasions, and determining that a wake-up signal (WUS) should be transmitted by the node to the communications device in advance of each of the one or more paging occasions which comprise the downlink messages for the communications device to decode, determining that a time since a most recent transmission of a signal which can be used by the communications device to re-synchronise with the node is greater than a predetermined threshold and transmitting, in response to determining that the time since the most recent transmission from the infrastructure equipment to the communications device is greater than the predetermined threshold, a preamble signal to the communications device.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.16/646,145, filed Mar. 11, 2020, which is based on PCT filingPCT/EP2018/075788, filed Sep. 24, 2018, which claims priority to EP17193861.6, filed Sep. 28, 2017, the entire contents of each areincorporated herein by reference.

BACKGROUND Field of Disclosure

The present disclosure relates to infrastructure equipment andcommunications devices of wireless communications systems, whereinfrastructure equipment are configured to transmit Wake-Up Signals(WUSs) in advance of transmitting downlink messages to communicationsdevices.

The present application claims the Paris Convention priority of Europeanpatent application EP17193861.6, the contents of which are herebyincorporated by reference.

Description of Related Art

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly or implicitly admitted as prior art against the presentinvention.

Third and fourth generation mobile telecommunication systems, such asthose based on the 3GPP defined UMTS and Long Term Evolution (LTE)architecture, are able to support more sophisticated services thansimple voice and messaging services offered by previous generations ofmobile telecommunication systems. For example, with the improved radiointerface and enhanced data rates provided by LTE systems, a user isable to enjoy high data rate applications such as mobile video streamingand mobile video conferencing that would previously only have beenavailable via a fixed line data connection. The demand to deploy suchnetworks is therefore strong and the coverage area of these networks,i.e. geographic locations where access to the networks is possible, maybe expected to increase ever more rapidly.

Future wireless communications networks will be expected to routinelyand efficiently support communications with a wider range of devicesassociated with a wider range of data traffic profiles and types thancurrent systems are optimised to support. For example it is expectedfuture wireless communications networks will be expected to efficientlysupport communications with devices including reduced complexitydevices, machine type communication (MTC) devices, high resolution videodisplays, virtual reality headsets and so on. Some of these differenttypes of devices may be deployed in very large numbers, for example lowcomplexity devices for supporting the “The Internet of Things” and maytypically be associated with the transmissions of relatively smallamounts of data with relatively high latency tolerance.

In view of this there is expected to be a desire for future wirelesscommunications networks, for example those which may be referred to as5G or new radio (NR) system/new radio access technology (RAT) systems,as well as future iterations/releases of existing systems, toefficiently support connectivity for a wide range of devices associatedwith different applications and different characteristic data trafficprofiles.

One example area of current interest in this regard includes theso-called “The Internet of Things”, or IoT for short. The 3GPP hasproposed in Release 13 of the 3GPP specifications to developtechnologies for supporting narrowband (NB)-IoT and so-called enhancedMTC (eMTC) operation using a LTE/4G wireless access interface andwireless infrastructure. More recently there have been proposals tobuild on these ideas in Release 14 of the 3GPP specifications withso-called enhanced NB-IoT (eNB-IoT) and further enhanced MTC (feMTC),and in Release 15 of the 3GPP specifications with so-called furtherenhanced NB-IoT (feNB-IoT) and even further enhanced MTC (efeMTC). See,for example, [1], [2], [3], [4]. At least some devices making use ofthese technologies are expected to be low complexity and inexpensivedevices requiring relatively infrequent communication of relatively lowbandwidth data.

The increasing use of different types of terminal devices associatedwith different traffic profiles gives rise to new challenges forefficiently handling communications in wireless telecommunicationssystems that need to be addressed.

SUMMARY OF THE DISCLOSURE

The present disclosure can help address or mitigate at least some of theissues discussed above.

Embodiments of the present technique can provide a method of operatingan infrastructure equipment in a wireless communications system. Thewireless communications system comprises the infrastructure equipmentand a communications device, and the method comprises detecting thatdownlink messages for the communications device to decode should betransmitted by the infrastructure equipment in one or more of aplurality of temporally spaced paging occasions, and determining that awake-up signal, WUS, should be transmitted by the infrastructureequipment to the communications device in advance of each of the one ormore paging occasions which comprise the downlink messages for thecommunications device to decode, determining that a time since a mostrecent transmission of a signal which can be used by the communicationsdevice to re-synchronise with the infrastructure equipment is greaterthan a predetermined threshold and transmitting, in response todetermining that the time since the most recent transmission from theinfrastructure equipment to the communications device is greater thanthe predetermined threshold, a preamble signal to the communicationsdevice, the preamble signal for use by the communications device as asynchronisation signal for the communications device to re-synchroniseits timing with the infrastructure equipment.

In some embodiments the preamble signal provides an indication to thecommunications device to go to sleep, whereby the communications devicecan reduce an amount of power consumer by its receiver, or to wake-up,whereby the communications device applied power to its receiver toreceiver signals within a paging time window.

Further embodiments of the present technique can provide a method ofoperating an infrastructure equipment in a wireless communicationssystem. The wireless communications system comprises the infrastructureequipment and a communications device, and the method comprisesdetecting that downlink messages for the communications device to decodeshould be transmitted by the infrastructure equipment during one or moreof a plurality of temporally spaced paging time windows, each pagingtime window comprising one or more of a plurality of temporally spacedpaging occasions, and determining that a wake-up signal, WUS, should betransmitted by the infrastructure equipment to the communications devicein advance of each of the one or more paging occasions which comprisethe downlink messages for the communications device to decode andtransmitting, in advance of every N of the paging time windows, where Nis an integer which equals one or more, a preamble signal to thecommunications device, the preamble signal for use by the communicationsdevice as a synchronisation signal for the communications device tore-synchronise its timing with the infrastructure equipment.

Embodiments of the present technique, which further relate toinfrastructure equipment, communications devices, methods of operatingcommunications devices and infrastructure equipment and circuitry forcommunications devices and infrastructure equipment, allow for thenetwork to transmit a preamble whenever there is inactivity for a longperiod of time, whilst also using the WUS prior to a PO for which thereis a potential paging message. This preamble acts as a synchronisationsignal for the UE to re-sync with the eNodeB.

Respective aspects and features of the present disclosure are defined inthe appended claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, but are notrestrictive, of the present technology. The described embodiments,together with further advantages, will be best understood by referenceto the following detailed description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings wherein likereference numerals designate identical or corresponding, partsthroughout the several views, and wherein:

FIG. 1 schematically represents some aspects of a LTE-type wirelesstelecommunication system which may be configured to operate inaccordance with certain embodiments of the present disclosure;

FIG. 2 schematically represents some aspects of a new radio accesstechnology (RAT) wireless telecommunications system which may beconfigured to operate in accordance with certain embodiments of thepresent disclosure:

FIGS. 3 and 4 schematically represent time lines associated with pagingoccasions in wireless telecommunication systems based on knownapproaches;

FIG. 5 schematically represents a first example format for wake upsignalling (WUS) that may be adapted for use in accordance with certainembodiments of the disclosure;

FIG. 6 schematically represents a second example format for wake upsignalling (WUS) that may be adapted for use in accordance with certainembodiments of the disclosure.

FIG. 7 represents an example of paging time windows comprising one ormore paging occasions;

FIG. 8 shows a part schematic, part message flow diagram representationof a communications system in accordance with embodiments of the presenttechnique;

FIG. 9 shows a flow diagram illustrating a process of communications ina communications system in accordance with embodiments of the presenttechnique;

FIG. 10 illustrates a first example implementation of GUS and WUStransmissions in accordance with embodiments of the present technique;

FIG. 11 illustrates a second example implementation of GUS and WUStransmissions in accordance with embodiments of the present technique,where preambles are transmitted in between paging time windows;

FIG. 12 illustrates a third example implementation of GUS and WUStransmissions in accordance with embodiments of the present technique,where the GUS indicates active or inactive paging time windows;

FIG. 13 illustrates a fourth example implementation of GUS and WUStransmissions in accordance with embodiments of the present technique,where the GUS indicates a portion of paging time windows that may beactive; and

FIG. 14 illustrates a fifth example implementation of GUS and WUStransmissions accordance with embodiments of the present technique,where the GUS indicates a portion of paging time windows that may beactive, where the paging occasions in that active region are controlledby WUSs.

DETAILED DESCRIPTION OF THE EMBODIMENTS Long Term Evolution AdvancedRadio Access Technology (4G)

FIG. 1 provides a schematic diagram illustrating some basicfunctionality of a mobile telecommunications network/system 10 operatinggenerally in accordance with LTE principles, but which may also supportother radio access technologies, and which may be adapted to implementembodiments of the disclosure as described herein. Various elements ofFIG. 1 and certain aspects of their respective modes of operation arewell-known and defined in the relevant standards administered by the3GPP® body, and also described in many books on the subject, forexample, Holma H. and Toskala A [5], It will be appreciated thatoperational aspects of the telecommunications networks discussed hereinwhich are not specifically described (for example in relation tospecific communication protocols and physical channels ha communicatingbetween different elements) may be implemented in accordance with anyknown techniques, for example according to the relevant standards andknown proposed modifications and additions to the relevant standards.

The network 10 includes a plurality of base stations 11 connected to acore network 12. Each base station provides a coverage area 13 (i.e. acell) within which data can be communicated to and from terminal devices14. Data is transmitted from base stations 11 to terminal devices 14within their respective coverage areas 13 via a radio downlink. Data istransmitted from terminal devices 14 to the base stations 11 via a radiouplink. The core network 12 routes data to and from the terminal devices14 via the respective base stations 11 and provides functions such asauthentication, mobility management, charging and so on. Terminaldevices may also be referred to as mobile stations, user equipment (UE),user terminal, mobile radio, communications device, and so forth. Basestations, which are an example of network infrastructureequipment/network access node, may also be referred to as transceiverstations/nodeBs/e-nodeBs/eNBs/g-nodeBs/gNBs and so forth. In this regarddifferent terminology is often associated with different generations ofwireless telecommunications systems for elements providing broadlycomparable functionality. However, certain embodiments of the disclosuremay be equally implemented in different generations of wirelesstelecommunications systems, and for simplicity certain terminology maybe used regardless of the underlying network architecture. That is tosay, the use of a specific term in relation to certain exampleimplementations is not intended to indicate these implementations arelimited to a certain generation of network that may be most associatedwith that particular terminology.

New Radio Access Technology (5G)

FIG. 2 is a schematic diagram illustrating a network architecture for anew RAT wireless mobile telecommunications network/system 30 based onpreviously proposed approaches which may also be adapted to providefunctionality in accordance with embodiments of the disclosure describedherein. The new RAT network 30 represented in FIG. 2 comprises a firstcommunication cell 20 and a second communication cell 21. Eachcommunication cell 20, 21, comprises a controlling node (centralisedunit) 26, 28 in communication with a core network component 31 over arespective wired or wireless link 36, 38. The respective controllingnodes 26, 28 are also each in communication with a plurality ofdistributed units (radio access nodes/remote transmission and receptionpoints (TRPs)) 22, 24 in their respective cells. Again, thesecommunications may be over respective wired or wireless links. Thedistributed units 22, 24 are responsible for providing the radio accessinterface for terminal devices connected to the network. Eachdistributed unit 22, 24 has a coverage area (radio access footprint) 32,34 which together define the coverage of the respective communicationcells 20, 21. Each distributed unit 22, 24 includes transceivercircuitry 22 a, 24 a for transmission and reception of wireless signalsand processor circuity 22 b, 24 b configured to control the respectivedistributed units 22, 24.

In terms of broad top-level functionality, the come network component 31of the new RAT telecommunications system represented in FIG. 2 may bebroadly considered to correspond with the core network 12 represented inFIG. 1 , and the respective controlling nodes 26, 28 and theirassociated distributed units/TRPs 22, 24 may be broadly considered toprovide functionality corresponding to base stations of FIG. 1 . Theterm network infrastructure equipment/access node may be used toencompass these elements and more conventional base station typeelements of wireless telecommunications systems. Depending on theapplication at hand the responsibility for scheduling transmissionswhich are scheduled on the radio interface between the respectivedistributed units and the terminal devices may lie with the controllingnode/centralised unit and or the distributed units/TRPs.

A terminal device 40 is represented in FIG. 2 within the coverage areaof the first communication cell 20. This terminal device 40 nits thusexchange signalling with the first controlling node 26 in the firstcommunication cell via one of the distributed units 22 associated withthe first communication cell 20. In some cases communications for agiven terminal device are routed through only one of the distributedunits, but it will be appreciated in some other implementationscommunications associated with a given terminal device may be routedthrough more than one distributed unit, for example in a soft handoverscenario and other scenarios. The particular distributed unit(s) throughwhich a terminal device is currently connected through to the associatedcontrolling node may be referred to as active distributed units for theterminal device. Thus the active subset of distributed units for aterminal device may comprise one or more than one distributed unit(TRP). The controlling node 26 is responsible for determining which ofthe distributed units 22 spanning the first communication cell 20 isresponsible for radio communications with the terminal device 40 at anygiven time (i.e. which of the distributed units are currently activedistributed units for the terminal device). Typically this will be basedon measurements of radio channel conditions between the terminal device40 and respective ones of the distributed units 22. In this regard, itwill be appreciated the subset of the distributed units in a cell whichare currently active for a terminal device will depend, at least inpart, on the location of the terminal device within the cell (since thiscontributes significantly to the radio channel conditions that existbetween the terminal device and respective ones of the distributedunits).

In at least some, implementations the involvement of the distributedunits in routing communications from the terminal device to acontrolling node (controlling unit) is transparent to the terminaldevice 40. That is to say, in some cases the terminal device may not beaware of which distributed unit is responsible for routingcommunications between the terminal device 40 and the controlling node26 of the communication cell 20 in which the terminal device iscurrently operating. In such cases, as far as the terminal device isconcerned, it simply transmits uplink data to the controlling node 26and receives down link data from the controlling node 26 and theterminal device has no awareness of the involvement of the distributedunits 22. However, in other embodiments, a terminal device may be awareof which distributed unit(s) are involved in its communications.Switching and scheduling of the one or more distributed units may bedone at the network controlling node based on measurements by thedistributed units of the terminal device uplink signal or measurementstaken by the terminal device and reported to the controlling node viaone or more distributed units.

In the example of FIG. 2 , two communication cells 20, 21 and oneterminal device 40 are shown for simplicity, but it will of course beappreciated that in practice the system may comprise a larger number ofcommunication cells (each supported by a respective controlling node andplurality of distributed units) serving a larger number of terminaldevices.

It will further be appreciated that FIG. 2 represents merely one exampleof a proposed architecture for a new RAT telecommunications system inwhich approaches in accordance with the principles described herein maybe adopted, and the functionality disclosed herein may also be appliedin respect of wireless telecommunications systems having differentarchitectures.

Thus certain embodiments of the disclosure as discussed herein may beimplemented in wireless telecommunication systems/networks according tovarious different architectures, such as the example architectures shownin FIGS. 1 and 2 . It will thus be appreciated the specific wirelesstelecommunications architecture in any given implementation is not ofprimary significance to the principles described herein. In this regard,certain embodiments of the disclosure may be described generally in thecontext of communications between network infrastructureequipment/access nodes and a terminal device, wherein the specificnature of the network infrastructure equipment/access node and theterminal device will depend on the network infrastructure for theimplementation at hand. For example, in some scenarios the networkinfrastructure equipment/access node may comprise a base station, suchas an LTE-type base station 11 as shown in FIG. 1 which is adapted toprovide functionality in accordance with the principles describedherein, and in other examples the network infrastructure equipment maycomprise a control unit/controlling node 26, 28 and/or a TRP 22, 24 ofthe kind shown in FIG. 2 which is adapted to provide functionality inaccordance with the principles described herein.

As is well understood, various wireless telecommunications networks,such as the LTE-based network represented in FIG. 1 and the NR-basednetwork represented in FIG. 2 , may support different Radio ResourceControl (RRC) modes for terminal devices, typically including: (i) RRCidle mode (RRC_IDLE); and (ii) RRC connected node (RRC_CONNECTED). Whena terminal device transmits data, RRC connected mode is generally used.The RRC idle mode, on the other hand, is for terminal devices which areregistered to the network (EMM-REGISTERED), but not currently in activecommunication (ECM-IDLE). Thus, generally speaking, in RRC connectedmode a terminal device is connected to a radio network access node (e.g.an LTE base station) in the sense of being able to exchange user planedata with the radio network access node. Conversely, in RRC idle mode aterminal device is not connected to a radio network access node in thesense of not being able to communicate user plane data using the radionetwork access node. In idle mode the terminal device may still receivesome communications from base stations, for example reference signallingfor cell reselection purposes and other broadcast signalling. The RRCconnection setup procedure of going from RRC idle mode to RRC connectedmode may be referred to as connecting to a cell/base station.

For a terminal device in RRC idle mode the core network is aware thatthe terminal device is present within the network, but the radio accessnetwork (RAN) part (comprising radio network infrastructure equipmentsuch as the base stations 11 of FIG. 1 and/or the combined TRPs/CUs ofFIG. 2 ) is not. The core network is aware of the location of idle modeterminal devices at a paging tracking area level but not at the level ofindividual transceiver entities. The core network will generally assumea terminal device is located within the tracking area(s) at with atransceiver entity most recently used for communicating with theterminal device, unless the terminal device has since provided aspecific tracking area update (TAU) to the network. (As is conventional,idle mode terminal devices are typically required to send a TAU whenthey detect they have entered a different tracking area to allow thecore network to keep track of their location.) Because the core networktracks terminal devices at a tracking area level, it is generally notpossible for the network infrastructure to know which specifictransceiver entities (radio network node) to use when seeking toinitiate contact with a terminal device in idle mode. Consequently, andas is well known, when a core network is required to connect to an idlemode terminal device a paging procedure is used.

In a typical currently deployed network, idle mode terminal devices areconfigured to monitor for paging messages periodically. For terminaldevices operating in a discontinuous reception (DRX) mode this occurswhen they wake up for their DRX awake time. Paging signals for aspecific terminal device are transmitted in defined frames (PagingFrames)/sub-frames (Paging Occasions) which for a given terminal devicemay be derived from the International Mobile Subscriber Identifier(IMSI) of the terminal device, as well as paging related DRX parametersestablished in system information transmitted within the network.

In a conventional system terminal device thus receives and checks thecontents of specific sub-frames (paging occasions) in specific frames(paging frames) to look for paging signalling. For example, inaccordance with the standards set out in 3GPP TS 36.304 version 14.2.0Release 14 [6], a Paging Frame (PF) is a downlink radio frame which maycontain one or more Paging Occasion(s) (PO), where a Paging Occasion isa sub-frame where there may be P-RNTI transmitted on PDCCH (orequivalent channel depending on implementation, e.g. on MPDCCH for MTCor for NB-IOT on NPDCCH) addressing the paging message. Paging messagesare conveyed on a physical downlink shared channel (PDSCH) on resourcesidentified from an allocation message addressed to a paging radionetwork temporary identifier (P-RNTI) and conveyed on a physicaldownlink control channel (PDCCH). P-RNTI is a common identifier for allterminal devices (e.g. set at FFFE in hexa-decimal for the standarddefined by 3GPP TS 36.321 version 13.5.0 Release 13 [7]). All terminaldevices check whether PDCCH at specific PFs/POs configured for their useinclude P-RNTI or not. If there is a PDSCH allocation addressed toP-RNTI in the relevant subframe the terminal device proceeds to seek toreceive and decode the paging messages transmitted on the allocatedresources on PDSCH. The UE then checks the list of IDs contained in thepaging record list in the received paging message, to determine whetherthe list contains an ID corresponding to itself (for example P-TMSI orIMSI), and if so initiates a paging response.

Although the above description has summarised an example existing LTEpaging procedure, it is expected that broadly similar principles may beadopted for future wireless telecommunications networks based on newerradio access technologies (RATs), such as 5G networks. Theabove-description of a paging procedure has referred to specific channelnames which are commonly used in LTE, such as PDCCH and PDSCH, and thisterminology will be used throughout this description for convenience, itbeing appreciated that in certain implementations different channelnames may be more common. For example in the context of a wirelesstelecommunications system having dedicated channels for communicatingwith certain types of terminal devices, for example MTC devices, it maybe expected the corresponding channel names may be modified. Forexample, a physical downlink control channel dedicated for MTC devicesmay be referred to as MPDCCH and a corresponding physical downlinkshared channel for MTC devices may be referred to as MPDSCH.

In proposed approaches for eNB-IoT and feMTC in accordance with 3GPPrelease 14, a terminal device in DRX in idle mode is required to decodePDCCH (or equivalent downlink control channel for the specificimplementation at hand) to identify if there are resources scheduled onPDSCH (or equivalent downlink shared channel for the specificimplementation at hand) for a paging message during paging occasions inwhich the terminal device might receive a paging message.

FIG. 3 schematically represents a timeline of a paging occasion for aterminal device operating in a known wireless telecommunications system.In the example shown in FIG. 3 , one paging occasion is shown andextends from time t1 to t2. As is conventional, paging occasions for aterminal device will typically occur according to a regular repeatingschedule having regard to the terminal device's currently configured DRXcycle, Different terminal devices may have different DRX cycle lengths,and so have different times between paging occasions. For a terminaldevice having a relatively long DRX cycle/time between paging occasions,it is possible the terminal device will to some extent losesynchronisation with the radio network infrastructure equipment of thetelecommunications system between paging occasions. In this case it maybe helpful for the terminal device to wake up in advance of the pagingoccasion to allow it to synchronise to the wireless telecommunicationssystem prior to the paging occasion. An example of this is schematicallyshown in FIG. 3 in which the terminal device wakes up at time t0 so thatit can synchronise with the wireless telecommunication system in theperiod between times t0 and t1 so that it is able to monitor/detectMPDCCH during the configured paging occasion between t1 and t2. In thisregard, the process of synchronisation might in some cases only requirefine adjustments to frequency and/or timing tracking loops based ondetection of CRS (cell-specific reference symbols), e.g. when DRX cycles(times between paging occasions) are relatively short, or a moresignificant degree of synchronisation may be needed, for examplecomplete re-synchronisation by detecting PSS/SSS (primarysynchronisation signals/secondary synchronisation signals) as well asusing CRS, e.g. when DRX cycles (times between paging occasions) arerelatively long (such that the frequency and timing of the terminaldevice may become significantly offset relative to that of the radionetwork infrastructure).

Once the terminal device has re-synchronised to the network, it willmonitor MPDCCH to determine if there is a paging message, and if so willgo on to decode the PDSCH carrying the paging message in the usual way.If there is no paging message for the terminal device, the terminaldevice will go back to sleep (low power mode) until the next pagingoccasion. For certain types of terminal devices, such as MTC devices, itmay be expected that paging will occur relatively rarely (e.g. once perday for a smart utility meter), and so in many cases the terminal devicemay wake up and synchronise to the network to monitor MPDCCH by blinddecoding for a potential DCI that may schedule a PDSCH containing apaging message when in fact there is no DCI or paging message for theterminal device. This represents an undesirable “waste” of resources,for example battery power, for the terminal device.

Wake-Up Signal (WUS)

Proposed approaches for eNB-IoT and feMTC in accordance with 3GPPrelease 15 share several common objectives, and one of these objectivesis to reduce power consumption associated with monitoring for pagingmassages by introducing what is referred to as a wake-up signal (WUS)(e.g. of the type described in C. Hambeck, et al., “A 2.4 μW Wake-upReceiver for wireless, sensor nodes with −71 dBm sensitivity”, in IEEEProceeding International Symposium of Circuits and Systems (ISCAS),2011, pp. 534-537 [8], or of a type defined in a co-pending Europeanpatent application, with application number 17186065.3 [9]). Theproposed WUS is carried on a new physical channel and is intended toallow terminal devices to determine whether or not they need to actuallydecode MPDCCH in an upcoming paging occasion. That is to say, whereas inaccordance with previously proposed techniques a terminal device isrequired to decode MPDCCH during every paging occasion to determine ifthere is a paging message, and if so to decode PDSCH to determine if thepaging message is addressed to the terminal device, the WUS is insteadintended to indicate to the terminal device whether or not the nextpaging occasion contains a paging message that the terminal deviceshould decode. A WUS is transmitted at a pre-determined/derivable timein advance of a scheduled paging occasion such that a terminal deviceknows when to seek to receive a WUS and may contain relatively littleinformation so that it can be decoded quickly (as compared to the blinddecoding needed for MPDCCH). For example, in some implementations theWUS may include a one-bit indication of whether or not there will be apaging message transmitted in the upcoming paging occasion. If the WUSindicates the upcoming paging occasion does include a paging message,any terminal devices for which that paging occasion applies may proceedto decode the paging message as normal to determine if the pagingmessage is addressed to it. If the WUS indicates the upcoming pagingoccasion does not include any paging message, any terminal device forwhich that paging occasion applies can determine from this that it doesnot need to monitor for a paging message during the upcoming pagingoccasion, and so can, for example, return to a low power mode. In someimplementations the WUS may include an identifier for a terminal devicethat is going to be paged in the paging occasion. This identifier mayidentify an individual terminal device or may identify a group ofterminal devices. The WUS may include multiple identifiers for multipleterminal devices/groups. A terminal device which determines the WUS isassociated with an identifier that applies to it may proceed to decodethe paging message as normal. Conversely, a terminal device whichdetermines the WUS is not associated with an identifier that applies toit may determine from this that it does not need to monitor for a pagingmessage during the upcoming paging occasion and can, for example, returnto a low power mode. The WUS may also be encoded with a format thatenables low power decoding (e.g. the WUS may be a narrow bandwidthsignal that can be decoded with low power using a low sampling ratereceiver), and furthermore may be transmitted with a format that allowsreliable decoding even with relatively poor synchronisation.

FIG. 4 schematically represents a timeline for a paging occasion for aterminal device operating in a wireless telecommunications systememploying a WUS as proposed in association with 3GPP Release 15. In theexample shown in FIG. 4 , a paging occasion extends from time τ2 to τ3.As is conventional, the paging occasions will typically occur accordingto a regular repeating schedule having regard to the terminal device'scurrently configured DRX cycle.

As schematically indicated in FIG. 4 , a WUS is transmitted at apredetermined/derivable time τ1 in advance of the paging occasion toindicate there is a MPDCCH paging message transmission for a terminaldevice indicated by an identifier associated with the WUS (theidentifier could identify an individual terminal device or a group ofterminal devices). The WUS can indicate that the UE should wake up inorder to read the paging occasion for at least one of the followingreasons:

-   -   The UE is potentially paged in the paging occasion        -   WUS may carry a single bit signal indicating whether the            paging occasion is active or not    -   The UE is within a group of UEs, where at least one of the UEs        group is paged at the paging occasion        -   WUS indicates a UE group ID    -   The UE is paged at the paging occasion        -   WUS indicates the ID of the UE being paged    -   System Information (SI) has changed and UEs need to re-read the        SI        -   WUS indicates that SI has changed

If the paging occasion is not scheduled to include an MPDCCH and apaging message transmission for a terminal device, then a WUSidentifying that terminal device is not sent. Thus a terminal device maybe configured to seek to detect a WUS associated with an identifier forthe terminal device in advance of an upcoming paging occasion. If theterminal device detects a WUS associated with an identifier for itself,the terminal device can proceed to fine tune its frequency and timingtracking loops if required and blind detects for an MPDCCH between timesτ2 and τ3, followed by decoding of the PDSCH carrying the paging messagebetween time τ3 and τ4 in the usual way. If, however, the terminaldevice fails to detect a WUS associated with an identifier for theterminal device, the terminal device may assume there is not going to bea paging message for the terminal device in the upcoming pagingoccasion, and so may go back to sleep (low power mode) and not decodeMPDCCH in the paging occasion. As noted above, in some otherimplementations, the WUS might not include any indication of anyspecific terminal device(s)/group(s), but may instead simply include anindication of whether or not an upcoming paging occasion includes anypaging message. Either way, by using WUS, a terminal device may beexpected to consume less energy as it can help avoid unnecessarymonitoring/blind decoding of MPDCCH (or equivalent depending on thespecific implementation at hand). It will be appreciated that WUS canalso be used in connected mode when DRX is used.

If a terminal device is configured for a long DRX cycle (i.e. relativelylong time between paging occasions), there may be a significantlikelihood the terminal device will lose synchronisation with the radioaccess network so that it is unable to decode WUS without firstsynchronising to the radio access network. An example of this isschematically shown in FIG. 4 whereby a terminal device configured for arelatively long DRX cycle may need to wake up at time τ0 to allow timefor it to synchronise to the radio access network before τ1 so that itcan detect any WUS signalling. Current proposals for feMTCsynchronisation rely on using PSS/SSS in the same way as for LTE.Because PSS/SSS signalling is typically relatively sparse, e.g. onlytwice per 10 ms radio frame in LTE, a terminal device needing tosynchronise to the network using this general synchronisation signallingis required to start doing so a relatively long time in advance of anexpected WUS occasion. This is exacerbated for terminal devices relyingon coverage enhancement techniques for reliable communications becauseof poor radio coverage (e.g. because the terminal device is located in abasement), since coverage enhancement techniques typically rely onaggregating repeat transmissions, meaning the terminal device needs towake up even sooner to receive a sufficient number of the transmissionsbefore an expected WUS occasion. This can mean terminal devicesconfigured for relatively long DRX cycles can consume relatively largeamounts of energy at each paging occasion, and would reduce or evenoffset the power saving benefit from using WUS.

To help address this issue with existing schemes relying on generalsynchronisation signalling, such as PSS/SSS in an LTE context, which istransmitted relatively infrequently, it has been proposed to transmitadditional synchronisation signalling in association with WUSsignalling. See, for example, N. S. Mazloum, O. Edfors, “PerformanceAnalysis and Energy Optimization of Wake-Up Receiver Schemes forWireless Low-Power Applications”, IEEE Transaction on WirelessCommunications, December 2014 [10]. In particular, it has been proposedin a co-pending European patent application, with application number17169577.8 [11] to transmit WUS signalling with synchronisationsignalling (e.g. a predetermined/derivable preamble/signature sequence)that can be used by terminal devices to synchronise to the network, forexample using conventional correlator techniques.

FIG. 5 schematically represents an example format for wake up signals(WUS) that include a synchronisation preamble (predefined signaturesequence) as described in [11]. Thus the WUS represented in FIG. 5comprises a preamble part 50, which may be of a variable length, and aninformation (“Info”) part 52. The preamble part 50 comprises signallingfor terminal devices to use to achieve frequency and/or timesynchronisation with the network (i.e. with the radio networkinfrastructure equipment transmitting the WUS), rather than using thesparsely distributed PSS/SSS (though it should be appreciated that thiscan still be optionally used in addition to the WUS preamble). Theinformation part 52 comprises an indication of one or more terminaldevices to which the WUS applies, e.g. a terminal device identifierand/or an identifier for a group of terminal devices. The terminaldevice/group identifier(s) may be network allocated identifiers (e.g.radio network temporary identifiers, RNTI) for the terminal device(s),or any other form of suitable identifier, e.g. based on an IMSI for aterminal device.

By providing additional/dedicated synchronisation signalling inassociation with the WUS, a terminal device may achieve synchronisationwith the network using this additional synchronisation signallingtransmitted contemporaneously with/around the same time as the WUS,rather than needing to rely on existing general synchronisationsignalling, which may be transmitted relatively infrequently in thewireless telecommunications system and so require the terminal device toexit a low power/sleep mode for an extended duration to achievesynchronisation to monitor for WUS signalling.

It will be appreciated there are many modifications that may be made tothe approaches set out above in relation to FIG. 5 , as described inco-pending European patent application no. 17169577.8, [11], thecontents of which are hereby incorporated by reference. For example, thegeneral format for the wake-up signalling may not conform to that shownin FIG. 5 , but may have a different format. For example, the wake-upsignalling may have a format such as shown in FIG. 6 in which thewake-up signalling comprises a preamble part 60 without a separateinformation part, and instead, the preamble may itself contain anindication of the identity for the terminal device(s) for which thewake-up signalling indicates a paging message is to be subsequentlytransmitted.

The WUS's preamble sequence would need to meet some of the followingrequirements:

-   -   It needs to be relatively short in time tea allow for a quick        synchronization and minimize battery power usage;    -   It needs to be possible to detect the WUS in a non-coherent        manner;    -   It needs to be detectable in very poor radio conditions, i.e.        SNR=−23 dB, to support coverage enhancement; and    -   It needs to be able to carry some information, which is useful        for a preamble only type of WUS, as in FIG. 6 .

WUS sequence designs which meet these requirements are taught byco-pending European patent application no. 17186065.3 [9], the contentsof which are hereby incorporated by reference.

In wireless communications systems discussed herein in relation toembodiments of the present technique, three different error cases arerelevant. These are:

-   -   Misdetection—A signal is transmitted, but no signal is received.    -   Incorrect detection—A signal is transmitted, but the signal is        received incorrectly. For example, a “1” is transmitted and a        “0” is received.    -   False alarm—No signal is transmitted, but a signal is received.

If the WUS is misdetected (i.e. a WUS is transmitted but the UE fails todetect it), then the UE would miss the corresponding paging message, andso the reliability of the paging is reduced. To avoid misdetection,another Power Saving Signal is proposed, where this signal is alwaystransmitted prior to a paging occasion (PO) and would indicate to the UEwhether it should Go To Sleep (i.e. there is no need for the UE tomonitor for MPDCCH and PDSCH) or Wake Up (i.e. monitor for MPDCCH andPDSCH in the corresponding PO). This Go to sleep or wake Up Signal(GUS), which is known and proposed in [12], would therefore remove anymisdetection since the UE would expect it to be there. The UE will missa paging occasion if there is an incorrect detection at the UE; i.e. theUP mistakes a Wake Up for a Go To Sleep indication. The drawback ofusing GUS is that it consumes a lot of resources, since it needs to betransmitted regardless of whether there is any potential paging messagefor the UE.

Extended DRX (eDRX) is introduced in Rel-13 for LTE to enable IoTdevices to sleep longer, thereby saving power, where the PO cycle isincreased from 2.56 seconds to 2621.44 seconds (256 Hyper-frames) foreMTC and 10485.76 seconds (1024 Hyper-frames) for NB-IoT. For a PO cyclethat is larger than 5.12 seconds, a Paging Time Window (PTW) is used. APTW is shown in FIG. 7 which has a time period of T_(L) and consists ofa burst of POs (four POs in this example) followed by deep sleep with aDRX cycle of T_(PTW). Each PTW, consists of one or more POs with a cycleof T_(PO), where in each PO the UP wakes up (active period) to monitorfor possible paging messages.

The internal clock used by IoT devices to keep in sync with the eNodeBtypically drifts during a DRX period. Assuming the worst case, where theclock drifts in OBC direction (either positive or negative), the longerthe DRX the further the UE's sync drifts away from that of the eNodeB.An example is, for an IoT device using a RTC (Real Time Clock) with adrift of 20 ppm, a T_(PTW) of 128 Hyper-frames (1310.72 seconds) wouldhave drifted by 26.2 ms or about 2.5 radio frames. Since a WUS is onlytransmitted if there is a potential paging message for the UE, theinactive time of the UE may be multiples of T_(PTW) if there is nopaging messages for a long time. That is to say, the drift is compoundedover several inactive PTW cycles, thereby leading to a larger drift.

One problem with having a large timing drift is that the UE needs tosearch for a WUS during the whole of the timing drift period. Thisextended searching window has two ill effects:

-   -   The WUS receiver needs to be active for the whole timing drift        period, increasing power consumption, and    -   It is possible to receive a false alarm at any time during the        timing drift window. Hence an extended timing drift window        increases the false alarm rate.

Embodiments of the present technique provide methods and apparatus whichcan reduce the timing drift window, while not requiring the base stationto send a GUS before every paging occasion (where transmission of a GUSconsumes resources). Though [12] discusses GUS, and WUS is widely known,there has been no proposal or discussion of the combined use of GUS andWUS under an eDRX operation, as proposed by embodiments of the presenttechnique.

Power Saving Signal Transmissions in eDRX

Embodiments of the present technique allow for the network to transmit apreamble (or more generally, a signal that allows the UE to establishsynchronisation with the network) whenever there is inactivity for along period of time, whilst also using the WUS prior to a PO for whichthere is a potential paging message. This preamble acts as asynchronisation signal for the UE to re-sync with the eNodeB.

FIG. 8 shows a part schematic, part message flow diagram representationof a communications system 80 in accordance with embodiments of thepresent technique. The communications system 80 comprises aninfrastructure equipment 81 and a communications device 82. Each of theinfrastructure equipment 81 and communications device 82 comprise atransmitter (or transmitter circuitry) 81.1, 82.1, a receiver (orreceiver circuitry) 81.2, 82.2 and a controller (or controllercircuitry) 83.1, 83.2. Each of the controllers 83.1, 83.2 may be, forexample, a microprocessor, a CPU, or a dedicated chipset, etc. It willbe appreciated by those skilled in the art that, in arrangements of thepresent technique, the communications device 82 may not always include atransmitter 82.1, for example in scenarios where the communicationsdevice 82 is a low-power wearable device.

As shown in FIG. 8 , the infrastructure equipment 81 is configured todetect 84 that downlink messages for the communications device 82 todecode should be transmitted by the infrastructure equipment 81 in oneor more of a plurality of temporally spaced paging occasions, and that awake-up signal, WUS, should be transmitted by the infrastructureequipment 81 to the communications device 82 in advance of each of theone or more paging occasions which comprise the downlink messages forthe communications device to decode, to determine 86 that a time since amost recent transmission of a signal for use by the communicationsdevice to re-synchronise its timing with the infrastructure equipmentfrom the infrastructure equipment 81 to the communications device 82 isgreater than a predetermined threshold, and to transmit 88, in responseto determining 86 that the time since the most recent transmission fromthe infrastructure equipment 81 to the communications device 82 isgreater than the predetermined threshold, a preamble signal to thecommunications device 82, the preamble signal for use by thecommunications device 82 as a synchronisation signal for thecommunications device 82 to re-synchronise its timing with theinfrastructure equipment 81. In some embodiments of the presenttechnique, the infrastructure equipment 81 may be an eNodeB, and thedetection 84 of the downlink messages for transmission to thecommunications device 82 may be in response to receiving commands from amobility management entity, the infrastructure equipment 81 (eNodeB) andthe mobility management entity both forming part of the same mobilecommunications network.

FIG. 9 shows a flow diagram illustrating a method of operating aninfrastructure equipment in a wireless communications system comprisingthe infrastructure equipment and a communications device in accordancewith embodiments of the present technique. The flow diagram of FIG. 9corresponds to the part schematic, part message flow diagramrepresentation of a communications system in accordance with embodimentsof the present technique. The method starts in step S90. The methodcomprises in step S92, detecting that downlink messages for thecommunications device to decode should be transmitted by theinfrastructure equipment in one or more of a plurality of temporallyspaced paging occasions, and determining that a wake-up signal, WUS,should be transmitted by the infrastructure equipment to thecommunications device in advance of each of the one or more pagingoccasions which comprise the downlink messages for the communicationsdevice to decode. In step S94, the process comprises determining, that atime since a most recent transmission of a signal for use by thecommunications device to re-synchronise its timing with theinfrastructure equipment from the infrastructure equipment to thecommunications device is greater than a predetermined threshold. In stepS96, the method comprises transmitting, in response to determining thatthe time since the most recent transmission from the infrastructureequipment to the communications device is greater than the predeterminedthreshold, a preamble signal to the communications device, the preamblesignal for use by the communications device as a synchronisation signalfor the communications device to re-synchronise its timing with theinfrastructure equipment. The process ends in step S98.

In some embodiments of the present technique, the preamble is a GUS,i.e. a GUS is transmitted at the start of every N PTW cycles and withina PTW a WUS is used prior to every PO. In other words, the downlinkmessages should be transmitted during one or more as plurality of pagingtime windows, each paging time window comprising one or more of thepaging occasions, each of the paging time windows being spaced by a timegreater than the predetermined threshold. Also, in other words, each WUScomprises an indication to the communications device of whether or notthe paging occasion which the each WUS precedes comprises a downlinkmessage for the communications device to decode. Additionally, one ormore of the preamble signals are go-to-sleep or wake-up signals, GUSs.In other words, in these embodiments, the method comprises transmitting,by the infrastructure equipment, one of the GUSs immediately beforeevery N of the paging time windows, where N is an integer which equalsone or more.

An example is shown in FIG. 10 where N=1, i.e. a GUS is transmittedprior to every PTW and within the PTW a WUS may be transmitted prior toeach PO. Although in FIG. 10 the WUS is shown to be transmitted prior toevery PO, it should be appreciated that this is just an example and thatin actual operation the WUS is only transmitted if there is a potentialpaging message for one or more UEs in the corresponding PO. Note that aWUS is not monitored by the UE prior to the first PO in the PTW sincethe GUS would indicate whether that PO is active (contains pagingmessage for at least one UE) or inactive. Since a GUS is transmittedregardless of whether there is a potential paging message in thecorresponding PO, this method would allow the use of the GUS as asynchronisation signal by the UE when it is deemed that the UE may havedrilled beyond an intolerable threshold. These embodiments of thepresent technique also recognise that GUSs consume resources and bylimiting them to the start of the PTWs, the resources used areminimised. Since WUSs are only transmitted if a PU is active, then thismethod would optimise resources if the portion of active POs is small(which is expected for IoT services) compared to using GUSs. Furthermorethe factor N can be used by the eNodeB to manage the resources used forGUS and the level of drift between eNodeB and UE (assuming the driftrate is known from specifications).

In other words, a broader level, in FIG. 10 , the Power Saving Signaltransmitted prior to every N PTWs has a different characteristic thanthose transmitted prior to the remaining POs within the PTW. Here theGUS transmitted prior to the PTW has a different characteristic that aWUS that are transmitted in remaining POs within the PTW.

Another difference in characteristic is that the length Power SavingSignal transmitted prior to every N PTWs is different to thosetransmitted prior to the remaining POs within that PTW. That is, in theexample in FIG. 10 , the length of the GUS (for example the number ofrepetitions of a primitive GUS sequence) may be greater than the lengthof the WUS. This is advantageous since there may be a large timing driftbetween PTW and synchronisation requires a greater amount of signalenergy when the timing drift is larger, where signal energy is providedby repetition/lengthening of the GUS signals. Since the timing drift isreduced to a small amount after reception of the GUS, the WUS signalsprior to each following paging occasion may be of shorter duration thanthe GUS (i.e. of shorter duration than the preamble signal prior to thePTW), thus saving on the resources required to transmit the WUS. Inother words, a number of repetitions of the first reference sequence ofthe GUS (or preamble signal) is different to a number of repetitions ofthe second reference sequence of the WUS, or a length of the preamblesignal is different to a length of the WUS.

In some embodiments, the length (for example represented by a number ofrepetitions of a primitive or base sequence) of a WUS or GUS depends onthe time since the communications device could have last previouslysynchronised. In other words a length of the preamble signal isdependent on a time since the most recently transmitted preamble signalwas transmitted from the infrastructure equipment to the communicationsdevice.

In some embodiments of the present technique, the preamble istransmitted in between two PTWs. If a GUS is used as a preamble then theGUS would indicate “Go To Sleep” in such intermediate preambles. This isfor cases where the eDRX, cycle is too long and hence transmitting apreamble between two PTW would allow the UE to achieve synchronisationwith the network and hence reduces the drift when its PTW arrives. Anexample is shown in FIG. 11 , where prior to each PTW a GUS istransmitted and within a PTW a WUS may be transmitted prior to each POdepending on whether there is any potential paging message. In betweentwo PTW, a preamble is transmitted for synchronisation purposes. Itshould be appreciated that this preamble can also be a GUS (set toindicate Go To Sleep) and that more than one preamble can be transmittedbetween two PTWs if the PTWs are too far apart.

In some embodiments of the present technique, the said preamble or GUSthat is transmitted prior to the start of a PTW would indicate whetherthe entire PTW contains any potential paging message to the UE. That is,the preamble or GUS would indicate whether the UE can ignore all the POswithin the corresponding PTW or whether it needs to wake up and monitora WUS for each PO within that PTW. A Wake Up is indicated if at leastone of the POs are active, i.e. contains a paging message addressing atleast one of the UEs in the group (note it is expected that the GUS andWUS signals are for a group of UEs and not all UEs would be paged),otherwise if none of the POs are active the GUS indicates Go To Sleep.An example is shown in FIG. 12 , where a GUS is transmitted prior toeach PTW. At time t₀, a GUS is transmitted and the corresponding PTW has2 POs that have paging messages for at least one UE within the group ofUEs. Here the GUS indicates a Wake Up to the UE (or group of UEs). Attime t₁ a Go To Sleep is indicated by the GUS since no PO is active(i.e. no PO contains a paging message) for any UE within the group ofUEs. It should be appreciated that the PO may be active for other groupsof UEs and so a separate GUS would be used to indicate paging activityto those UEs. It should be noted that a WUS is monitored by the UE fromthe first PO in the PTW since the GUS indicates whether there are anyactive POs rather than whether a specific PO is active or not. In otherwords, each GUS comprises an indication to the communications device ofwhether or not one or more of the paging occasions in the paging timewindow which the each GUS precedes comprise a downlink message for thecommunications device to decode. In other words, each GUS indicates thatthe communications device should wake up if one or more of the pagingoccasions in the paging time window which each GUS precedes comprise adownlink message for the communications device to decode, or each GUSindicates that the communications device should go to sleep if none ofthe paging occasions in the paging time window which each GUS precedescomprise a downlink message for the communications device to decode.

In some embodiments of the present technique, a GUS could indicatewhether any POs are active within a portion of the PTW, i.e. for exampleif a PTW contains 8 POs, the GUS can indicate whether the first 4 POsare active. The UE then needs to monitor a WUS or possibly another GUSfor the remaining POs in the PTW. An example is shown in FIG. 13 , wherea PTW contains 8 POs. At time t₀ a GUS is transmitted and indicates thatthe first 4 POs are inactive thereby transmitting a Go To Sleep signalto the group of UEs, i.e. each goes to sleep from time t₁ till t₂ andwill not monitor for anything. However, this group of UEs would have tomonitor for possible WUS after the 4^(th) PO at time t₀. At time t3, aGUS is transmitted and indicates a Wake Up since one of the second 4 POsin the corresponding PTW is active. Here, the UE would have to monitorfor WUS for the rest of the PTW. As described, a separate GUS can betransmitted at time t₅ to indicate that the last 4 POs are inactive. Thenumber of POs that the GUS can indicate to be active or not can beconfigured by the network or specified in the specifications e.g. as apercentage of the total PO. This embodiment recognises that the eNBscheduler may not be able to schedule too far ahead and so for a PTWwith a large number of POs, it may not be practical or may even berestrictive for a GUS to indicate whether all the POs in the PTW areinactive ahead of time. It should be appreciated that although theexample in FIG. 13 shows that the GUS addresses only the first few POswithin a PTW, this embodiment is also applicable if the portion of thePTW addresses another subset of POs that may reside in the middle or theend of the PTW. In other words, each GUS comprises an indication to thecommunications device of whether or not one or more of the pagingoccasions in a subset of the paging time window which the each GUSprecedes comprise a downlink message for the communications device todecode.

In some embodiments of the present technique, a GUS can indicate whethera portion of the PTW is active and each PO within that portion iscontrolled by a WUS, as shown in FIG. 14 . This Figure shows:

-   -   t₀: GUS indicates “Go To Sleep” for the portion of PTW from t₁        to t₂. Hence the UE does not monitor paging occasions between t₁        and t₂    -   t₂: the UE monitors WUS to determine whether paging occasions        are active or not within the portion of the PTW that is not        controlled by GUS    -   t₃: GUS indicates “Wake Up” for the portion of the PTW from t₄        to t₅.    -   t₄ to t₅: UE monitors WUS before each paging, occasion and        decodes MPDCCH in the paging occasion if WUS indicates that the        associated PO is active    -   t₅: the UE monitors WUS to determine whether paging occasions        are active or not within the portion of the PTW that is not        controlled by GUS

As described in [9], the WUS may comprise N_(p) WUS preamble symbolsfollowed by N_(d) WUS signalling symbols if needed. Each WUS OFDMpreamble symbol comprises 3 components:

-   -   Pseudo-random Sequence (PN)    -   Zadoff-Chu (ZC) sequence    -   Frequency shift function.

The dot product of the PN and ZC sequences forms the WUS preamblesequence. Each WUS preamble symbol is constructed as an OFDM symbol inthe frequency domain by mapping the elements of the WUS preamblesequence to the designated subcarriers of the OFDM symbol. For each WUSpreamble OFDM symbol, the PN sequence is initialized by the Cell ID sothat a WUS is linked to the cell and this would also randomize the WUSso that it is orthogonal to the WUS from other cells. The use of ZCsequences is meant to provide good correlation properties at the UEreceiver and low PAPR for the transmitted signal. The WUS preamblesequence for preamble symbol m is X_(m)(k) is:X _(m)(k)=P _(m)(k)Z(k)where, P_(m)(k) is the PN sequence, Z(k) is the Zadoff-Chu sequence fork=0, 1, 2, . . . N_(SC)−1 where N_(SC) is the number of sub-carriersused for the WUS signal.

The WUS preamble symbol is then formed by:w _(m)(k)=x _(m)(k)e ^(−ja) ^(m) ^(k)where x_(m)(k) is the inverse Fourier transform of X_(m)(k) and a_(m) isa frequency shift component for preamble symbol m.

In embodiments of the present technique, both a GUS and a WUS may beformed in the same way as described above in relation to the disclosureof [9]. Furthermore, in some embodiments, the WUS sequence may be one ofthe sequences used by the GUS. As described above, in [9] a Tower SavingSignal sequence is described, where in embodiments of the presenttechnique the sequence can be used as a WUS or GUS. When it is used as aGUS, a different ZC root or a different PN or a different frequencyshift can be used to distinguished between a Go To Sleep indication anda Wake Up indication. The GUS may be constructed as two derivations of aPower Saving Signal sequence, and the WUS is one of such derivations.For example the GUS consists of the ZC sequence with a root of q_(s)used to indicate “Go Sleep” whilst a ZC sequence with root q_(w) is usedto indicate “Wake Up”. The corresponding WUS would therefore use a ZCwith a root of q_(s). Such embodiments of the present techniquetherefore minimise the number of sequences required for GUS and WUS,which would reduce the eNodeB and UE complexities in managing, thenumber of required sequences. In other words, in such embodiments, eachGUS comprises a preamble formed by a plurality of Orthogonal FrequencyDivision Multiplexed, OFDM, symbols, each of the OFDM symbols beingmodulated with a first reference sequence intended to indicate a Wake Upto the UE, or a second reference sequence intended to indicate a Go ToSleep to the UE and wherein one or more of the OFDM symbols aretransmitted having been shifted in frequency with respect to successiveothers of the OFDM symbols. Each WUS comprises a preamble formed by aplurality of Orthogonal Frequency Division Multiplexed, OFDM, symbols,each of the OFDM symbols being modulated with a reference sequence, thereference sequence being the first of reference sequences used for theGUS, and wherein one or more of the OFDM symbols are transmitted havingbeen shifted in frequency with respect to successive others of the OFDMsymbols.

In some embodiments of the present technique, different ZC roots of aGUS can be used to indicate different potential subsets of active or inPOs. In other words, the first reference sequence is dependent onwhether or not one or more of the paging occasions in the paging timewindow which each GUS precedes or in a subset of the paging time windowwhich each GUS precedes comprise a downlink message for thecommunications device to decode.

The Applicant's co-pending patent applications published underpublications numbers US 2017/026219 A1 [13], US 2017/026220 A1 [14] andUS 2017/026221 A1 [15] each disclose bootstrap signals for digitaltelevision, where the bootstrap signals comprise OFDM symbols which arecombined with reference sequences. The reference sequences are acombination of PN and ZC sequences. However, the disclosures of theseco-pending patent applications are not appropriate for describing WUSsor GUSs, as described by embodiments of the present technique, becausethere are no prefix and postfix OFDM symbols as are present in thebootstrap signals for digital television. The contents of US 2017/026219A1 [13], US 2017/026220 A1 [14] and US 2017/026221 A1 [15] are eachincorporated herein by reference.

Those skilled in the art would appreciate that such infrastructureequipment and/or communications devices as herein defined may be furtherdefined in accordance with the various arrangements and embodimentsdiscussed in the preceding paragraphs. It would be further appreciatedby those skilled in the art that such infrastructure equipment andcommunications devices as herein defined and described may form part ofcommunications systems other than those defined by the presentinvention.

The following numbered paragraphs provide further example aspects andfeatures of the present technique:

Paragraph 1. A method of operating an infrastructure equipment in awireless communications system comprising the infrastructure equipmentand a communications device, wherein the method comprises:

-   -   detecting that downlink messages for the communications device        to decode should be transmitted by the infrastructure equipment        in one or more of a plurality of temporally spaced paging        occasions, and determining that a wake-up signal, WUS, should be        transmitted by the infrastructure equipment to the        communications device in advance of each of the one or more        paging occasions which comprise the downlink massages for the        communications device to decode;    -   determining that a time since a most recent transmission of a        signal which can be used by the communications device to        re-synchronise with the infrastructure equipment is greater than        a predetermined threshold; and    -   transmitting, in response to determining that the time since the        most recent transmission from the infrastructure equipment to        the communications device is greater than the predetermined        threshold, a preamble signal to the communications device, the        preamble signal for use by the communications device as a        synchronisation signal for the communications device to        re-synchronise its timing with the infrastructure equipment.

Paragraph 2. A method according to Paragraph 1, wherein the downlinkmessages should be transmitted during one or more of a plurality ofpaging time windows, each paging time window comprising one or more ofthe paging occasions, each of the paging time windows being spaced by atime greater than the predetermined threshold.

Paragraph 3. A method according to Paragraph 1 or 2, wherein one or moreof the preamble signals are go-to-sleep or wake-up signals, GUSs.

Paragraph 4. A method according to Paragraph 3, comprising

-   -   transmitting, by the infrastructure equipment, one of the GUSs        immediately before every N of the paging time windows, where N        is an integer which equals one or more.

Paragraph 5. A method according to Paragraph 3 or 4, wherein each WUScomprises an indication to the communications device of whether or notthe paging, occasion which the each WUS precedes comprises a downlinkmessage for the communications device to decode.

Paragraph 6. A method according to Paragraph 4 or 5, wherein each GUScomprises an indication to the communications device of whether or notone or more of the paging occasions in the paging time window which theeach GUS precedes comprise a downlink message for the communicationsdevice to decode.

Paragraph 7. A method according to Paragraph 6, wherein the each GUSindicates that the communications device should wake up if one or moreof the paging occasions in the paging time window which the each GUSprecedes comprise a downlink message for the communications device todecode.

Paragraph 8. A method according to Paragraph 6, wherein the each GUSindicates that the communications device should go to sleep if none ofthe paging occasions in the paging time window which the each GUSprecedes comprise a downlink message for the communications device todecode.

Paragraph 9. A method according to Paragraph 4 or 5, wherein each GUScomprises an indication to the communications device of whether or notone or more of the paging occasions in a subset of the paging timewindow which the each GUS precedes comprise a downlink message for thecommunications device to decode.

Paragraph 10. A method according to any of Paragraphs 3 to 9, whereineach GUS comprises a preamble formed by a plurality of OrthogonalFrequency Division Multiplexed, OFDM, symbols, each of the OFDM symbolsbeing modulated with a first reference sequence, and wherein one or moreof the OFDM symbols are transmitted having been shifted in frequencywith respect to successive others of the OFDM symbols.

Paragraph 11. A method according to Paragraph 10, wherein each WUScomprises a preamble formed by a plurality of Orthogonal FrequencyDivision Multiplexed, OFDM, symbols, each of the OFDM symbols beingmodulated with a second reference sequence, the second referencesequence being a subset of the filet reference sequence, and wherein oneor more of the OFDM symbols are transmitted having been shifted infrequency with respect to successive others of the OFDM symbols.

Paragraph 12. A method according to Paragraph 10 or Paragraph 11,wherein the first reference sequence is dependent on whether or not oneor more of the paging occasions in the paging time window which the eachGUS precedes or in a subset of the paging time window which the each GUSprecedes comprise a downlink message for the communications device todecode.

Paragraph 13. A method according to Paragraph 11, wherein a number ofrepetitions of the first reference sequence of the GUS is different to anumber of repetitions of the second reference sequence of the WUS.

Paragraph 14. A method according to any of Paragraphs 1 to 13, wherein alength of the preamble signal is different to a length of the WUS.

Paragraph 15. A method according to any of Paragraphs 1 to 13, wherein alength of the preamble signal is dependent on a time since the mostrecently transmitted preamble signal was transmitted from theinfrastructure equipment to the communications device.

Paragraph 16. A method of operating an infrastructure equipment in awireless communications system comprising the infrastructure equipmentand a communications device, wherein the method comprises:

-   -   detecting that downlink messages for the communications device        to decode should be transmitted by the infrastructure equipment        during one or more of a plurality of temporally spaced paging        tune windows, each paging time window comprising one or more of        a plurality of temporally spaced paging occasions, and        determining that a wake-up signal, WUS, should be transmitted by        the infrastructure equipment to the communications device in        advance of each of the one or more paging occasions which        comprise the downlink messages for the communications device to        decode; and    -   transmitting, in advance of every N of the paging time windows,        where N is an integer which equals one or more, a preamble        signal to the communications device, the preamble signal for use        by the communications device as a synchronisation signal for the        communications device to re-synchronise its timing with the        infrastructure equipment.

Paragraph 17. A method according to Paragraph 16, wherein one or more ofthe preamble signals are go-to-sleep or wake-up signals, GUSs.

Paragraph 18. An infrastructure equipment for use in a wirelesscommunications system comprising the infrastructure equipment and acommunications device, the infrastructure equipment comprisingtransceiver circuitry and controller circuitry which are configured incombination

-   -   to detect that downlink messages for the communications device        to decode should be transmitted by the infrastructure equipment        in one or more of a plurality of temporally spaced paging        occasions, and to determine that a wake-up signal, WUS, should        be transmitted by the infrastructure equipment to the        communications device in advance of each of the one or more        paging occasions which comprise the downlink messages for the        communications device to decode;    -   to determine that a time since a most recent transmission of a        signal which can be used by the communications device to        re-synchronise with the infrastructure equipment is greater than        a predetermined threshold; and    -   to transmit, in response to determining that the time since the        most recent transmission from the infrastructure equipment to        the communications device is greater than the predetermined        threshold, a preamble signal to the communications device, the        preamble signal for use by the communications device as a        synchronisation signal for the communications device to        re-synchronise its timing with the infrastructure equipment.

Paragraph 19. Circuitry for an infrastructure equipment for use in awireless communications system comprising the infrastructure equipmentand a communications device, the infrastructure equipment comprisingtransceiver circuitry and controller circuitry which are configured incombination

-   -   to detect that downlink messages for the communications device        to decode should be transmitted by the infrastructure equipment        in one or more of a plurality of temporally spaced paging        occasions, and to determine that a wake-up signal, WUS, should        be transmitted by the infrastructure equipment to the        communications device in advance of each of the one or more        paging occasions which comprise the downlink messages for the        communications device to decode;    -   to determine that a time since a most recent transmission of a        signal which can be used by the communications device to        re-synchronise with the infrastructure equipment is greater than        a predetermined threshold; and    -   to transmit, in response to determining that the time since the        most recent transmission from the infrastructure equipment to        the communications device is greater than the predetermined        threshold, a preamble signal to the communications device, the        preamble signal for use by the communications device as a        synchronisation signal for the communications device to        re-synchronise its timing with the infrastructure equipment.

Paragraph 20. An infrastructure equipment for use in a wirelesscommunications system comprising the infrastructure equipment and acommunications device, the infrastructure equipment comprisingtransceiver circuitry and controller circuitry which are configured incombination

-   -   to detect that downlink messages for the communications device        to decode should be transmitted by the infrastructure equipment        during one or more of a plurality of temporally spaced paging        time windows, each paging time window comprising one or more of        a plurality of temporally spaced paging occasions, and to        determine that a wake-up signal, WUS, should be transmitted by        the infrastructure equipment to the communications device in        advance of each of the one or more paging occasions which        comprise the downlink messages for the communications device to        decode; and    -   to transmit, in advance of every N of the paging time windows,        where N is an integer which equals one or more, a preamble        signal to the communications device, the preamble signal for use        by the communications device as a synchronisation signal for the        communications device to re-synchronise its timing with the        infrastructure equipment.

Paragraph 21. Circuitry for an infrastructure equipment for use in awireless communications system comprising the infrastructure equipmentand a communications device, the infrastructure equipment comprisingtransceiver circuitry and controller circuitry which are configured incombination

-   -   to detect that downlink messages for the communications device        to decode should be transmitted by the infrastructure equipment        during one or more of a plurality of temporally spaced paging        time windows, each paging time window comprising one or more of        a plurality of temporally spaced paging occasions, and to        determine that a wake-up signal, WUS, should be transmitted by        the infrastructure equipment to the communications device in        advance of each of the one or more paging occasions which        comprise the downlink messages for the communications device to        decode; and    -   to transmit, in advance of every N of the paging time windows,        where N is an integer which equals one or more, a preamble        signal to the communications device, the preamble signal for use        by the communications device as a synchronisation signal for the        communications device to re-synchronise its timing with the        infrastructure equipment.

In so far as embodiments of the disclosure have been described as beingimplemented, at least in part, by software-controlled data processingapparatus, it will be appreciated that a non-transitory machine-readablemedium carrying such software, such as an optical disk, a magnetic disk,semiconductor memory or the like, is also considered to represent anembodiment of the present disclosure.

It will be appreciated that the above description for clarity hasdescribed embodiments with reference to different functional units,circuitry and/or processors. However, it will be apparent that anysuitable distribution of functionality between different functionalunits, circuitry and/or processors may be used without detracting fromthe embodiments.

Described embodiments may be implemented in any suitable form includinghardware, software, firmware or any combination of these. Describedembodiments may optionally be implemented at least partly as computersoftware running on one or more data processors and/or digital signalprocessors. The elements and components of any embodiment may bephysically, functionally and logically implemented in any suitable way.Indeed the functionality may be implemented in a single unit, in aplurality of units or as part of other functional units. As such, thedisclosed embodiments may be implemented in a single unit or may bephysically and functionally distributed between different units,circuitry and/or processors.

Although the present disclosure has been described in connection withsome embodiments, it is not intended to be limited to the specific formset forth herein. Additionally, although a feature may appear to bedescribed in connection with particular embodiments, one skilled in theart would recognise that various features of the described embodimentsmay be combined in any manner suitable to implement the technique.

REFERENCES

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What is claimed is:
 1. A method of operating a communications device ina wireless communications system comprising an infrastructure equipmentand the communications device, wherein the method comprises: decodingdownlink messages transmitted by the infrastructure equipment in one ormore of a plurality of temporally spaced paging occasions; decoding awake-up signal (WUS) transmitted by the infrastructure equipment inadvance of each of the one or more paging occasions which comprise thedownlink messages; and receiving, in response to determining that a timesince a most recent transmission from the infrastructure equipment tothe communications device is greater than a predetermined threshold, apreamble signal from the infrastructure equipment, and re-synchronizingwith the infrastructure equipment based on the preamble signal.
 2. Themethod of claim 1, wherein the downlink messages are received during oneor more of a plurality of paging time windows, each paging time windowcomprising one or more of the paging occasions, each of the pagingwindows being spaced by a time greater than the predetermined threshold.3. The method of claim 2, wherein one or more of the preamble signalsare go-to-sleep or wake-up signals (GUSs).
 4. The method of claim 3,further comprising: receiving, from the infrastructure equipment, one ofthe GUSs immediately before every N of the paging time windows, where Nis an integer which equals one or more.
 5. The method of claim 4,wherein each GUS comprises an indication of whether one or more of thepaging occasions in the paging time window which the each GUS precedescomprise a downlink message for the communications device to decode. 6.The method of claim 5, wherein the each GUS indicates that thecommunications device should wake up if one or more of the pagingoccasions in the paging time window which the each GUS precedes comprisea downlink message for the communications device to decode.
 7. Themethod of claim 5, wherein the each GUS indicates that thecommunications device should go to sleep if none of the paging occasionsin the paging time window which the each GUS precedes comprise adownlink message for the communications device to decode.
 8. The methodof claim 3, wherein each WUS comprises an indication of whether thepaging occasion which the each WUS precedes comprises a downlink messagefor the communications device to decode.
 9. The method of claim 4,wherein each GUS comprises an indication to the communications device ofwhether or not one or more of the paging occasions in a subset of thepaging time window which the each GUS precedes comprise a downlinkmessage for the communications device to decode.
 10. The method of claim3, wherein each GUS comprises a preamble formed by a plurality ofOrthogonal Frequency Division Multiplexed (OFDM) symbols, each of theOFDM symbols being modulated with a first reference sequence, andwherein one or more of the OFDM symbols are transmitted having beenshifted in frequency with respect to successive others of the OFDMsymbols.
 11. The method of claim 10, wherein each WUS comprises apreamble formed by a plurality of OFDM symbols, each of the OFDM symbolsbeing modulated with a second reference sequence, the second referencesequence being a subset of the first reference sequence, and wherein oneor more of the OFDM symbols are transmitted having been shifted infrequency with respect to successive others of the OFDM symbols.
 12. Themethod of claim 11, wherein a number of repetitions of the firstreference sequence of the GUS is different than a number of repetitionsof the second reference sequence of the WUS.
 13. The method of claim 10,wherein the first reference sequence is dependent on whether one or moreof the paging occasions in the paging time window which the each GUSprecedes or in a subset of the paging time window which the each GUSprecedes comprise a downlink message for the communications device todecode.
 14. The method of claim 1, wherein a length of the preamblesignal is different than a length of the WUS.
 15. The method of claim 1,wherein a length of the preamble signal is dependent on a time since themost recently transmitted preamble signal was transmitted from theinfrastructure equipment to the communications device.
 16. Thecommunications device of claim 1, wherein the circuitry is configured toreceive the downlink messages during one or more of a plurality ofpaging time windows, each paging time window comprising one or more ofthe paging occasions, each of the paging windows being spaced by a timegreater than the predetermined threshold.
 17. The communications deviceof claim 16, wherein one or more of the preamble signals are go-to-sleepor wake-up signals (GUSs).
 18. The communications device of claim 17,wherein the circuitry is configured to receive, from the infrastructureequipment, one of the GUSs immediately before every N of the paging timewindows, where N is an integer which equals one or more.
 19. Thecommunications device of claim 17, wherein each WUS comprises anindication of whether the paging occasion which the each WUS precedescomprises a downlink message for the communications device to decode.20. A communications device comprising: circuitry configured to decodedownlink messages transmitted by infrastructure equipment of a wirelesscommunication system in one or more of a plurality of temporally spacedpaging occasions; decode a wake-up signal (WUS) transmitted by theinfrastructure equipment in advance of each of the one or more pagingoccasions which comprise the downlink messages; and receive, in responseto determining that a time since a most recent transmission from theinfrastructure equipment to the communications device is greater than apredetermined threshold, a preamble signal from the infrastructureequipment, and re-synchronizing with the infrastructure equipment basedon the preamble signal.